Human presence detection systems are primarily intended for use at secure portals where it is desirable to search vehicular traffic to assure that no one enters or leaves the secured area by concealing themselves in a vehicle. Usually, two or more sensors are placed on an apparently unoccupied vehicle, and a test is initiated by an operator. The system acquires data from the sensors, performs an analysis of the sensor data, and gives a “pass” or “search” result based on the analysis. Current systems operate with an acceptable degree of reliability when used in the absence of wind or in locations sheltered from the wind. As the level of ambient wind even slightly increases, the reliability of these systems decreases. The primary failure mechanism is an increase in the number of false search indications caused by the wind. This major limitation has been overcome in some applications by using the system indoors, or by erecting barriers to block the wind.
Either of these solutions can significantly increase the cost of using such a system. They also limit its portability, virtually eliminating the ability to use it in temporary “spot checks” at locations where a permanent installation is not practical.
Another limitation of current systems is the means for determining if the wind conditions are outside the range for which the system can operate reliably. In some systems, this determination is based solely on the statistical properties of the sensor data.
In others, a separate sensor is used to sense when wind conditions may not be favorable for analysis. While the use of a separate sensor is more reliable than using the statistical properties of the sensor data, it represents an additional expense to the system, and complicates the system setup and operation. It would be better if a way could be found to determine directly from the sensor data whether the current wind conditions are outside the range of reliable operation of the device.
The limitations of the current systems are due to the signal analysis methods that are used. They make a decision based on the amount of motion energy (power) detected in the input signals. They make no attempt to analyze the nature of the motion in the sensor input signals. The current devices are based on either time domain or frequency domain analysis. While the prior methods allow a quantitative determination of the total motion energy present in the signal, they do not attempt to identify any characteristic and potentially distinguishing features of the motion signature that would indicate whether the motion is due to the presence of a heartbeat or is due to the wind.
The present invention analyzes the types of motion present in the sensor signals and thereby significantly improves the range of ambient environmental conditions under which a human presence detection system can reliably operate.